So-called digital flat detectors for recording digital X-ray images of an object are known in X-ray imaging, in which X-ray radiation is converted by a scintillator layer or a direct converter layer to an electrical charge and is then electronically read by way of so-called active pixel matrices, for example composed of amorphous silicon (a-Si), subjected to analogue-digital conversion, and further-processed as a so-called X-ray raw image for image production.
Because of the specific characteristics of the flat detector, it is necessary in order to achieve an X-ray image that is as good as possible to subject the image information that has been read to post-processing. Offset corrections and gain corrections are typically carried out, as well as defect corrections and line noise corrections as well.
Offset correction compensates for temporary changes in the dark current behavior of the pixel elements, of which the active pixel matrix is composed. For this post-processing, a blank offset correction image is recorded without any X-ray radiation present, and this is subtracted in a known manner from a subsequent X-ray raw image, with the aid of an image processing method. The gain correction is used to compensate for sensitivity fluctuations from one pixel element to another. In this case, a known multiplication method is carried out on the X-ray image, which has already been subjected to offset correction, with the aid of a gain correction image.
Overall, the combined offset and gain correction, the so-called flat-field correction, can in general be described as follows: T=G·[S−O] and Tn=Gn·[Sn=On], respectively, where S represents the X-ray raw image, Sn represents the raw value related to the respective pixel element n, O the offset correction image and On the offset value, G the gain correction image and Gn the gain value, T the corrected X-ray image and Tn the corrected final value. Nowadays, offset and gain corrections are normally linear corrections.
However, flat detectors image a very wide dynamic range, over which the flat detector is in general no longer linear; in addition, non-linearities vary locally, that is to say from one pixel element to another or from one pixel row to another. Particularly in the case of X-ray doses which are very much higher or very much lower than the dose in which, for example, the gain image was produced, the local non-linearities can lead to artifacts being visible in the linearly corrected X-ray image. Non-linear corrections are admittedly known, but they are virtually unusable because of the large amounts of data, particularly in the case of fluoroscopy applications, in which up to 30 X-ray images are produced per second.